U.S. Patent Application Publication No. 2006/0223137 discloses a reagent for partial lysis of a cell membrane of a red blood cell so as to enable a fluorescent dye to be transmitted while holding a malaria parasite inside the red blood cell. In the Examples, a description is made for detecting a malaria infected red blood cell using a measurement sample (concentration of Hoechst 34580: 1.80 μM) prepared by adding 2 μl of a dye solution (0.5 mg/ml), in which Hoechst 34580 as a fluorescent dye is dissolved with ethylene glycol, and 1 ml of the reagent for partial lysis of the cell membrane of the red blood cell to 20 μl of a specimen, and subjecting the measurement sample to a flow cytometer.